ARM: dts: porter: Fix HDMI output routing

The HDMI encoder is connected to the RGB output of the DU, which is
port@0, not port@1. Fix the incorrect DT description.

Fixes: c5af8a42 ("ARM: dts: porter: add DU DT support")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>